Method for removing waxes from molded part in powder injection molding by using mixed fluid

ABSTRACT

In a method for removing a wax material from a molded part, to remove a major binder, waxes, from a molded part fabricated by a powder injection molding method or a compression molding method, a supercritical mixed fluid of carbon dioxide and propane is used as a dewaxing solvent.

TECHNICAL FIELD

[0001] The present invention relates to a dewaxing method for removing a binder from a molded part in powder injection molding (PIM) process in order to produce solid products having a complicated shape in large quantities. In particular, the present invention relates to a dewaxing method for selectively removing a major binder of a low molecular group such as a wax material from a molded part fabricated by a powder injection molding method or a compression molding method, using a mixed fluid of carbon dioxide and propane, under the condition that a temperature is not less than the critical temperature and a pressure is not less than the critical pressure.

BACKGROUND ART

[0002] A powder injection molding method is a new powder molding process combining a powder metallurgy technique of the metal industry with an injection molding technique of the plastic industry. This process is appropriate to producing molded parts having a complicated shape in large quantities. In particular, it can be applied to cutting tools, magnetic material parts, noble metal parts and a MEMS (micro electro mechanical system), etc. Generally, in a molding process including a powder injection molding process, a dewaxing process is for removing a binder added for improving a fluidity of fine solid

[0003] Hereinafter, the conventional dewaxing method will be descried. In the conventional dewaxing method, lots of dewaxing time and energy are required. The conventional dewaxing method can be divided into a heating dewaxing method, a solvent dewaxing method and a catalyst dewaxing method. In relation to the heating dewaxing method, Wicking method was disclosed in U.S. Pat. No. 5,028,367 and U.S. Pat. No. 4,404,166. In relation to the solvent dewaxing method, a method using methylenechloride, acetone or freon, etc. as a solvent was disclosed in U.S. Pat. No. 4,197,118 and U.S. Pat. No. 4,765,950. In relation to the catalyst dewaxing method, a method using a catalyst such as nitric acid and boronfluoride (BF3), etc. was disclosed in U.S. Pat. No. 5,531,958 and U.S. Pat. No. 5,073,319.

[0004] Among the above conventional dewaxing methods, the heating dewaxing method requires dozens˜hundreds of time and a high temperature up to 300˜500°C., and accordingly it has defects in the temporal and economical aspects. In addition, in comparison with the heating dewaxing method, the solvent dewaxing method can reduce a dewaxing time to some degree. However, because an organic solvent harmful to environment and humans is used, usage thereof has been regulated, the regulation will be strengthened.

[0005] In general, because a supercritical fluid has a diffusion speed approaching to that of gas, it has good penetrating and carrying properties, and because it has a strong solvency owing to its density similar to that of liquid, it is very effective in extracting a binder from porous materials. In addition, because solvency is varied according to changes of temperature and pressure, it is possible to extract selectively a binder from materials. Using the above characteristics of supercritical fluid, U.S. Pat. No. 4,731,208 and Japan Patent No. 2000-144205 have disclosed a dewaxing process using supercritical carbon dioxide, in a metal powder injection molding method.

[0006] In the powder injection molding method, a mixture of waxes of a low molecular group and polyethylene, polystyrene, polypropylene, etc. of a high molecular group is generally used as a binder. Among the binder, the low molecular group waxes should be removed from a molded part in the dewaxing process, however, the high molecular group such as polyethylene, polystyrene and polypropylene, etc. should remain in the molded part after dewaxing, in order to combine metal powders with each other and maintain a shape of the molded part. But, there is a problem that those high molecular group materials are removed in a general sintering process.

[0007] The dewaxing method using supercritical carbon dioxide is for removing only a wax component of the low molecular group from the molded part. This method requires a dewaxing time shorter than that of the heating dewaxing method or the catalyst dewaxing method. In particular, a method for further reducing a dewaxing time by adding a co-solvent mixed with a nonpolar solvent such as n-hexane, methylenechloride, etc. to supercritical carbon dioxide was disclosed in Japan Patent No. 1992-120204.

SUMMARY OF THE INVENTION

[0008] In dewaxing processes of a powder injection molding method or a compression molding method, in order to solve the problems of the conventional solvent dewaxing method using an organic solvent harmful to environment and the conventional heating dewaxing method having defects in the economical aspects, it is an object of the present invention to provide a dewaxing method using a supercritical mixed fluid which is capable of dewaxing more quickly and economically. In addition, it is another object of the present invention to provide a dewaxing method using a two-component supercritical mixed fluid which is capable of easily adjusting solubility according to temperature and pressure not less than the critical point.

DETAILED DESCRIPTION OF THE INVENTION

[0009] In a dewaxing method in accordance with the present invention, in order to improve flowability of fine solid powders of a molded part obtained by a powder injection molding method or a compression molding method, only a major binder of a low molecular group such as waxes among the binders in the molded part is selectively removed under a temperature and pressure not less than the critical temperature and pressure, by using a mixed fluid of carbon dioxide and propane.

[0010] The present invention relates to a dewaxing method using a two-component supercritical mixed fluid of carbon dioxide and propane. Because of characteristics of propane having a very strong solubility to nonpolar material under the conditions of not less than the critical point, the two-component supercritical mixed fluid of carbon dioxide and propane excels a pure supercritical carbon dioxide in solubility. The dewaxing method of the present invention using the two-component supercritical mixed fluid of carbon dioxide and propane can efficiently reduce a dewaxing time in comparison with the conventional dewaxing method using a pure supercritical carbon dioxide or the conventional supercritical carbon dioxide dewaxing method added a co-solvent. It will be described in more detail as follows.

[0011] Among binders in a molded part, almost all waxes of a lower molecular group consist of saturated hydrocarbon having a linear molecular structure and have nonpolarity. The nonpolar saturated hydrocarbon is well dissolved in a nonpolar organic solvent, particularly it is more efficiently dissolved in supercritical propane than supercritical carbon dioxide. However, propane has a very high critical temperature as 96.6° C. Thus, when supercritical propane is applied to the dewaxing process of the powder injection molding method, because materials of a high molecular group such as polyethylene, polystyrene and polypropylene, etc. necessary for maintaining a shape of a molded part are melted, the shape of the molded part may be deteriorated, and accordingly it is impossible to use supercritical propane in the dewaxing process. In addition, because propane is combustible, its usage is restricted.

[0012] According to the present invention, after mixing carbon dioxide with propane to use a powerful solubility of propane and nonflammability of carbon dioxide, the process is performed under the conditions not less than the critical temperature and the critical pressure. In more detail, propane in the supercritical state has powerful solubility, however, due to flammability thereof it can not be used easily. In order to solve this problem, carbon dioxide as nonflammable gas is added and mixed with propane. Herein, the critical point of the mixed fluid is varied according to a mixture ratio of carbon dioxide and propane. By setting a temperature, at which the high molecular group such as polyethylene, polystyrene and polypropylene, etc. of the binders in the molded part are not dissolved, as a maximum process temperature, the critical point of the mixed fluid of carbon dioxide and propane is calculated, and the mixture ratio is adjusted to make the critical temperature of the mixed fluid lower than the above maximum process temperature. Accordingly, it is possible to remove only major binder from the molded part quickly, without changing shapes of the molded part.

[0013] In the present invention, using a known supercritical dewaxing apparatus [J. Am. Ceram. Soc., 78, 1787-1792, (1995)], a mixed fluid of supercritical carbon dioxide and propane with an appropriate mixture ratio was put in a cylinder of the dewaxing apparatus, and the mixed fluid was filled into a dewaxing container, using a high pressurizing liquid pump. The dewaxing container includes an inlet for injecting materials and a metal screen for mounting materials. When the molded part was dewaxed for a certain time under the conditions not less than the critical temperature and pressure by using the mixed fluid, a low molecular material of the binder in the molded part is dissolved, and the low molecular material is separated from the supercritical mixed fluid in a separator through a back-pressure regulator. In the meantime, carbon dioxide and propane respectively had a degree of purity as 99% and 99.5%.

[0014] In the above method, the molded part obtained by mixing tungsten carbide hard metal powder (WC-Ni) with the binder and injection-molding the mixture was used as a test sample. The injection-molded part, having a thickness of 3 mm, was used as a watchband, and a size of a particle of the tungsten carbide hard metal powder was 1.31 μm. In the binder according to the present invention, paraffin wax was used as a major binder, polyethylene was used as a minor binder, stearic acid was added as a surfactant, and the ratio of the major binder was 70 wt. %. Carbon dioxide-propane used as the supercritical mixed fluid was injected into the dewaxing container with the speed of a moving fluid of 2 g/min, the ratio of propane to the total mixed fluid was in the range of 5 wt. %˜85 wt. % (carbon dioxide wt. %:propane wt. %=95:5˜15:85), and it was used under the supercritical conditions. Herein, because 5 wt. % was a minimum concentration in the economical aspects, and 85 wt. % was a concentration having a maximum process temperature at which a binder of the high molecular group was not melted, the minimum and the maximum concentrations were defined as 5 wt. % and 85 wt. %, respectively.

[0015] The supercritical conditions will be described in detail with reference to following Table 1. TABLE 1 Mixture Ratio (wt %) Critical Temperature Critical Pressure Carbon Dioxide Propane (° C.) (bar) 95  5 32.9 75.8 90 10 35.1 76.7 80 20 40.7 76.5 70 30 47.1 73.9 60 40 54.0 69.8 50 50 61.2 65.2 40 60 68.5 60.4 30 70 75.7 55.7 20 80 82.0 51.2 15 85 86.4 49.0

[0016] Based on Table 1, it is preferable to perform the dewaxing process of the present invention at a temperature in the range of 32.9˜86.4° C. and under pressure not less than 100 bar, preferably in the range of 100˜500 bar.

[0017] Hereinafter, the present invention will be better understood from the below examples, but those examples are given only to illustrate the present invention, not to limit the scope of the invention.

EXAMPLE 1

[0018] In order to remove a paraffin wax material from a molded part fabricated by the metal powder injection molding method, in substitute for the conventional supercritical carbon dioxide dewaxing method, a supercritical mixed fluid of carbon dioxide-propane was used.

[0019] After injecting a test sample of 3 mm thickness made from tungsten carbide hard metal powder by the injection molding method, into a SUS 316 dewaxing container having a capacity of 300 cc, the mixed fluid of 90 wt. % carbon dioxide-10 wt. % propane was pressed by a high-pressurizing liquid pump as 250 bar pressure, and a temperature was maintained as 85° C. When the mixed fluid reaches the supercritical state, the paraffin wax began to be removed from the test sample, after 50 minutes, all paraffin wax as a major binder was removed from the test sample.

Example 2

[0020] By the same method as Example 1, a dewaxing process was performed by using a mixed fluid of 70 wt. % carbon dioxide-30 wt. % propane under the pressure of 250 bar and the temperature of 75° C. As a result, all paraffin wax as a major binder was removed from the test sample in 10 minutes.

Example 3

[0021] By the same method as Example 1, a dewaxing process was performed by using a mixed fluid of 70 wt. % carbon dioxide-30 wt. % propane under the pressure of 150 bar and the temperature of 75° C. As a result, all paraffin wax as a major binder was removed from the test sample in 45 minutes.

Example 4

[0022] By the same method as Example 1, a dewaxing process is performed by using a mixed fluid of 50 wt. % carbon dioxide-50 wt. % propane under the pressure of 250 bar and the temperature of 75° C. As a result, all paraffin wax as a major binder was removed from the test sample in 5 minutes.

Comparative Example 1

[0023] The dewaxing time required in the conventional wicking method (one of heating dewaxing methods) was compared with that of the supercritical mixed fluid dewaxing method in accordance with Example 1 of the present invention.

[0024] The same test sample as Example 1 was injected into a cast containing alumina powder, and was heated at a temperature to 300° C. with a heat rising speed of 1.14° C./min. The heating was continued for 2 hours at 300° C. and evaporated paraffin wax was gradually removed from the test sample by a capillary force of the allumina powder. To remove all the paraffin wax from the test sample by the conventional heating dewaxing method, about 10 hours were required. In more detail, a time 60 times larger than that of Example 2 using a mixed fluid of 70 wt. % carbon dioxide-30 wt. % propane was required.

Comparative Example 2

[0025] The dewaxing time required in the conventional supercritical carbon dioxide dewaxing method was compared with that of the supercritical mixed fluid dewaxing method in accordance with Example 2 of the present invention.

[0026] By using the same test sample as Example 1, a dewaxing process is performed under the conditions of supercritical carbon dioxide, 250 bar pressure at a temperature of 75° C. All paraffin wax as a major binder was removed from the test sample in 2.5 hours. Accordingly, under the same pressure and temperature conditions, the dewaxing time of Example 2 using a mixed fluid of 70 wt. % carbon dioxide-30 wt. % propane was reduced more than 15 times in comparison with that of the conventional method using only supercritical carbon dioxide.

[0027] Dewaxing test results of Examples 1˜4 and Comparative Examples 1, and 2 will be described in detail with reference to following Table 2. TABLE 2 Propane Carbon Dioxide Dewaxing Dewaxing Dewaxing Content Content Temperature Pressure Time Classification (wt. %) (wt. %) (° C.) (bar) (minute)* Example 1 10 90 85 250 50 Example 2 30 70 75 250 10 Example 3 30 70 75 150 45 Example 4 50 50 75 250  5 Comparative — — 300   1 600  Example 1 Comparative — 100  75 250 150  Example 2

[0028] Industrial Applicability

[0029] In the present dewaxing method using a mixed fluid of carbon dioxide-propane, a dewaxing process can be performed in a time shorter than that of the conventional dewaxing method. By varying the propane content in a mixed fluid, the operation temperature and pressure, the dewaxing time can be more efficiently reduced. Accordingly, the present invention can make an improvement in the dewaxing process of a powder injection molding method used for producing solid products having three-dimensional complicated shape in large quantities. 

What is claimed is:
 1. A dewaxing method for removing was material of a binder from a molded part fabricated by a powder injection molding method or a compression molding method, wherein a supercritical mixed fluid of carbon dioxide and propane is used as a dewaxing solvent.
 2. The method of claim 1, wherein the ratio of carbon dioxide to propane (carbon dioxide wt. %: propane wt. %) in the supercritical mixed fluid is 95 wt. %:5 wt. %˜15 wt. %:85 wt. %.
 3. The method of claim 1, wherein a dewaxing temperature is in the range of 32.9˜86.4° C.
 4. The method of claim 1, wherein a dewaxing pressure is in the range of 100˜500 bar. 